The present invention relates to a new and distinctive watermelon mutant allele, designated “HMBN”. There are numerous steps in the development of any novel, desirable plant germplasm. Plant breeding begins with the analysis and definition of problems and weaknesses of the current germplasm, the establishment of program goals, and the definition of specific breeding objectives. The next step is selection of germplasm that possess the traits to meet the program goals. The goal is to combine in a single variety or hybrid an improved combination of desirable traits from the parental germplasm. These important traits may include higher yield, field performance, fruit and agronomic quality such as firmness, color, content in soluble solids, acidity and viscosity, resistance to diseases and insects, and tolerance to drought and heat.
All cultivated forms of watermelon belong to the polymorphic species Citrullus lanatus that is grown for its edible fruit that weigh from five to forty pounds, depending on variety. As a crop, watermelons are grown commercially wherever environmental conditions permit the production of an economically viable yield. In the United States, the main watermelon production areas are Florida, Georgia, Texas and California. Fresh watermelon are eaten sliced or diced and may also be used as an ingredient in prepared foods.
Watermelon is thought to have originated in southern Africa because it is found growing wild throughout the area, and reaches maximum diversity of forms there. It has been cultivated in Africa for over 4,000 years. Citrullus colocynthis is considered to be a wild ancestor of watermelon. It has fruit that are small, with a maximum diameter of 75 mm, with bitter flesh and small, brown seeds. Although Citrullus species grow wild in southern and central Africa, C. colocynthis also grows wild in India. Cultivation of watermelon began in ancient Egypt and India and is though to have spread from those countries through the Mediterranean, Near East, and Asia. The crop has been grown in the Untied States since 1629.
Citrullus lanatus is a member of the family Cucurbitaceae which consists of about 90 genera and 700 to 760 species, mostly of the tropics. The family includes pumpkins, squashes, gourds, watermelon, loofah, and several weeds. There are four recognized Citrullus species, C. lanatus, C. colocynthis, C. rehmii and C. ecirrhosus; all have 22 chromosomes and can be crossed with each other successfully.
C. lanatus is an annual watermelon. It has large, broad green leaves, which are orbicular to triangular-ovate in shape and deeply three to five lobed or sometimes simple. Medium-sized flowers are monoecious and have short pedicels. Fruits are of medium to large size, with thick rind and solid flesh with high water content. Flesh color may be red, yellow, or white. Seeds are ovate to oblong, are strongly compressed and have white or brown seed coats. The root system of the plant is a deep, spreading fibrous semi-taproot system that extends six feet or more below the soil surface.
C. colocynthis is a perennial watermelon. It differs from C. lanatus primarily in the size of plant organs. Leaves are small with narrow lobes, and are hairy and grayish in color. Flowers are monoecious and small. Bloom is profuse in autumn, when fresh vegetative growth also occurs. Seeds are small and brown. Fruits are small, not exceeding 3 inches in diameter, with rind and spongy flesh that are always bitter.
C. ecirrhosus is a perennial watermelon. C. ecirrhosus closely resembles C. colocynthis in vegetative characteristics, but its leaves are more divided, are covered with dense fine hairs, and have strongly recurved margins. Tendrils are lacking. Fruits are subglobose with white flesh and are bitter like C. colocynthis. Flowers are not produced until the second year of growth.
Commercial watermelon plants are monoecious, producing both male and female flowers. A female flower can be easily recognized by the swelling of its base that resembles a tiny watermelon. Honeybees, mainly in the morning, pollinate the flowers. There are many diverse cultivars for production with varieties having dark green to yellow rind coloring, striped or solid coloring, and containing seeds or are seedless. The shape of the fruit varies from round to elliptical.
Watermelon varieties fall into three broad classes based on how the seed was developed: open-pollinated, F1 hybrid and triploid (commonly referred to as seedless). Open-pollinated varieties are developed through several generations of selection. The selection can be based upon yield, quality characteristics and disease resistance. F1 hybrids are developed from two inbred lines that have been selfed for several generations and then crossed. F1 hybrid seed exhibit increased uniformity of type and time of harvest compared with open-pollinated seed and can exhibit as much as a 20 percent to 40 percent increase in yields over open-pollinated varieties grown under similar conditions. The third type is triploid or seedless watermelon. These are developed by creating watermelon plants with double the usual chromosome number and crossing them with normal watermelon plants. The resulting plants have one-and-a-half times the normal chromosome number. Because they have an odd number of chromosomes, they cannot form viable seed. Although triploid watermelons are referred to as seedless, they are not truly seedless but rather have undeveloped seeds that are soft and edible.
Choice of breeding or selection methods depends on the mode of plant reproduction, the heritability of the trait(s) being improved, and the type of cultivar used commercially (e.g., F1 hybrid cultivar, pureline cultivar, etc.). For highly heritable traits, a choice of superior individual plants evaluated at a single location will be effective, whereas for traits with low heritability, selection should be based on mean values obtained from replicated evaluations of families of related plants. Popular selection methods commonly include pedigree selection, modified pedigree selection, mass selection, and recurrent selection.
The complexity of inheritance influences choice of the breeding method. Backcross breeding is used to transfer one or a few favorable genes for a highly heritable trait into a desirable cultivar. This approach has been used extensively for breeding disease-resistant cultivars. Various recurrent selection techniques are used to improve quantitatively inherited traits controlled by numerous genes. The use of recurrent selection in self-pollinating crops depends on the ease of pollination, the frequency of successful hybrids from each pollination, and the number of hybrid offspring from each successful cross.
Each breeding program should include a periodic, objective evaluation of the efficiency of the breeding procedure. Evaluation criteria vary depending on the goal and objectives, but should include gain from selection per year based on comparisons to an appropriate standard, overall value of the advanced breeding lines, and number of successful cultivars produced per unit of input (e.g., per year, per dollar expended, etc.).
Promising advanced breeding lines are thoroughly tested and compared to appropriate standards in environments representative of the commercial target area(s) for three years at least. The best lines are candidates for new commercial cultivars; those still deficient in a few traits are used as parents to produce new populations for further selection.
These processes, which lead to the final step of marketing and distribution, usually take from eight to 12 years from the time the first cross is made. Therefore, development of new cultivars is a time-consuming process that requires precise forward planning, efficient use of resources, and a minimum of changes in direction.
A most difficult task is the identification of individuals that are genetically superior, because for most traits the true genotypic value is masked by other confounding plant traits or environmental factors. One method of identifying a superior plant is to observe its performance relative to other experimental plants and to a widely grown standard cultivar. If a single observation is inconclusive, replicated observations provide a better estimate of its genetic worth.
The goal of watermelon plant breeding is to develop new, unique and superior watermelon inbreds and hybrids. The breeder initially selects and crosses two or more parental lines, followed by repeated selfing and selection, producing many new genetic combinations. The breeder can theoretically generate billions of different genetic combinations via crossing, selfing and mutations. The breeder has no direct control at the cellular level. Therefore, two breeders will never develop the same line, or even very similar lines, having the same watermelon traits.
Each year, the plant breeder selects the germplasm to advance to the next generation. This germplasm is grown under unique and different geographical, climatic and soil conditions, and further selections are then made, during and at the end of the growing season. The varieties which are developed are unpredictable. This unpredictability is because the breeder's selection occurs in unique environments, with no control at the DNA level (using conventional breeding procedures), and with millions of different possible genetic combinations being generated. A breeder of ordinary skill in the art cannot predict the final resulting lines he develops, except possibly in a very gross and general fashion. The same breeder cannot produce the same line twice by using the exact same original parents and the same selection techniques. This unpredictability results in the expenditure of large research monies to develop superior new watermelon varieties.
The development of commercial watermelon hybrids requires the development of homozygous inbred lines, the crossing of these lines, and the evaluation of the crosses. Pedigree, backcross or recurrent selection breeding methods are used to develop lines from breeding populations. Breeding programs combine desirable traits from two or more lines or various broad-based sources into breeding pools from which mutant alleles are developed by selfing and selection of desired phenotypes. The new inbreds are crossed with other inbred lines and the hybrids from these crosses are evaluated to determine which have commercial potential.
Pedigree breeding is used commonly for the improvement of self-pollinating crops or inbred parents of cross-pollinating crops. Two parents which possess favorable, complementary traits are crossed to produce an F1. An F2 population is produced by selfing one or several F1's or by intercrossing two F1's (sib mating). Selection of the best individuals is usually begun in the F2 population; then, beginning in the F3, the best individuals in the best families are selected. Replicated testing of families, or hybrid combinations involving individuals of these families, often follows in the F4 generation to improve the effectiveness of selection for traits with low heritability. At an advanced stage of inbreeding (i.e., F6 and F7), the best lines or mixtures of phenotypically similar lines are tested for potential release as new cultivars or new parents for hybrids.
Mass and recurrent selections can be used to improve populations of either self- or cross-pollinating crops. A genetically variable population of heterozygous individuals is either identified or created by intercrossing several different parents. The best plants are selected based on individual superiority, outstanding progeny, or excellent combining ability. The selected plants are intercrossed to produce a new population in which further cycles of selection are continued.
Backcross breeding has been used to transfer genes for a simply inherited, highly heritable trait into a desirable homozygous cultivar or inbred which is the recurrent parent. The source of the trait to be transferred is called the donor parent. The resulting plant is expected to have the attributes of the recurrent parent (e.g., cultivar) and the desirable trait transferred from the donor parent. After the initial cross, individuals possessing the phenotype of the donor parent are selected and repeatedly crossed (backcrossed) to the recurrent parent. The resulting plant is expected to have the attributes of the recurrent parent (e.g., cultivar) and the desirable trait transferred from the donor parent.
Mutation breeding is another method of introducing new traits into watermelon varieties. Mutations that occur spontaneously or are artificially induced can be useful sources of variability for a plant breeder. The goal of artificial mutagenesis is to increase the rate of mutation for a desired characteristic. Mutation rates can be increased by many different means including temperature, long-term seed storage, tissue culture conditions, radiation (such as X-rays, Gamma rays, neutrons, Beta radiation, or ultraviolet radiation), chemical mutagens (such as base analogues like 5-bromo-uracil), antibiotics, alkylating agents (such as sulfur mustards, nitrogen mustards, epoxides, ethyleneamines, sulfates, sulfonates, sulfones, or lactones), azide, hydroxylamine, nitrous acid or acridines. Once a desired trait is observed through mutagenesis the trait may then be incorporated into existing germplasm by traditional breeding techniques. Details of mutation breeding can be found in “Principles of Cultivar Development” by Fehr, Macmillan Publishing Company, 1993.
Mutation breeding includes chromosome doubling agents such as the chemical colchicine, which inhibits microtubule formation during cell division. When treated with colchicine, a cell's chromosomes are copied in preparation for mitosis as normal, but the lack of microtubules prevents cell cleavage. The result is an undivided cell that contains double the normal complement of the organism's chromosomes. The colchicine-treated cell is then regenerated into a full plant in which each cell has its chromosomes doubled. If an individual with mismatched chromosomes is treated with colchicine, its chromosomes will be doubled, thus creating a matching partner chromosome that is able to match up properly during sexual reproduction. The procedure can restore fertility to a formerly sterile individual and the newly fertile, amphidiploid plant can then produce segregating offspring that can be observed for further traits. Colchicine may also be used to double the chromosome number of a normal, cultivated plant so that the plant may be able to readily combine with another plant that has a different number of chromosomes.
Descriptions of other breeding methods that are commonly used for different traits and crops can be found in one of several reference books (e.g., Principles of Plant Breeding, John Wiley and Son, pp. 115-161, 1960; Allard, 1960; Simmonds, 1979; Sneep et al., 1979; Fehr, 1987).
Proper testing should detect any major faults and establish the level of superiority or improvement over current cultivars. In addition to showing superior performance, there must be a demand for a new cultivar that is compatible with industry standards or which creates a new market. The introduction of a new cultivar will incur additional costs to the seed producer, the grower, processor and consumer; for special advertising and marketing, altered seed and commercial production practices, and new product utilization. The testing preceding release of a new cultivar should take into consideration research and development costs as well as technical superiority of the final cultivar. For seed-propagated cultivars, it must be feasible to produce seed easily and economically.
Once the inbreds that give the best hybrid performance have been identified, the hybrid seed can be reproduced indefinitely as long as the homogeneity of the inbred parent is maintained. A single-cross hybrid is produced when two inbreds are crossed to produce the F1 progeny.
Watermelon has been improved by domestication and formal plant breeding from a late maturing vine with small fruit having hard, white flesh and bland or bitter taste, into an early maturing, more compact plant with large fruit having edible, sweet flesh. In the last century, plant breeders working in public or private programs in the United States and around the world have released varieties having disease resistance, dwarf vines, larger fruit, higher sugar content, higher lycopene content, seedlessness, and new flesh colors, such as dark red, orange and yellow. Recent advances in the breeding of seedless triploid hybrids have resulted in renewed popularity of watermelons, and per capita consumption has increased 37% since 1980.
However, even with such a tremendous diversity, most watermelon plants are large and produce large fruits weighing from five to forty pounds while there is an increasing demand for smaller plants and fruits. Some smaller plants have been discovered and a gene, dw-1, resulting in a dwarf plant habit has been identified as a single recessive gene (Mohr, H. C., Proc. Assoc. Southern Agric. Work., 53:174 (1956)). Another single recessive dwarfing gene, dw-2, which controls multibranching from the crown of the plant was identified in 1975 (Mohr, H. C. and M. S. Sandhu, J. Am. Soc. Hortic. Sci. 100:135-137).
These dwarfing genes apply only to the plant and not to the fruit resulting in large fruit on small plants. It has been very difficult for watermelon breeders to develop small plants with small fruit and commercially acceptable yield. Unexpectedly, the HMBN allele of the present invention results in both small plants and smaller fruits with commercially acceptable yield.